This invention relates to a composition of matter comprising partially debranched starches prepared by enzymatic debranching and to an enzymatic process for preparing the debranched starches.
Starch is a polysaccharide typically comprising a mixture of about 20-25% amylose and about 75-80% amylopectin which is organized into compact granular structures. Amylose is a linear polymer of D-anhydroglucose units which are linked by alpha-1,4-D-glucosidic bonds. Amylopectin is a large branched polymer of amylose chains linked by alpha-1,6-D-glucosidic bonds in a tree-like structure. Depending upon the variety of plant from which the starch is obtained, amylose ordinarily contains between 250 and 12,500 D-anhydroglucose units and amylopectin contains between 400,000 and 3,125,000 D-anhydroglucose units.
Enzymes, or mixtures of enzymes which saccharify and debranch starch, have been used in starch conversion processes for the commercial production of low molecular weight oligosaccharides and sugars, such as dextrose (glucose). Starch conversion is the degradation of starch to lower molecular weight components by treatment with acid, oxidizing agents, heat, alkali or alpha-amylase enzymes. Enzymatic conversion of starch typically involves preferential hydrolysis of the alpha-1,4-D-glucosidic bonds, and only limited, if any, hydrolysis of the alpha-1,6-D-glucosidic bonds.
In the enzymatic conversion of starch to thin-boiling (low viscosity) starch, hydrolysis of branched fragments may be incomplete. For sugar production, however, complete conversion of starch to sugar is desirable, and debranching enzymes have been used to degrade the branched alpha-limit dextrins (branched starch fragments which resist further hydrolysis by alpha-amylase) which remain intact after the enzymatic hydrolysis of alpha-1,4-D-glucosidic bonds. Glucoamylase, an enzyme which liquifies and saccharifies starch, has been employed for this purpose. Glucoamylase rapidly hydrolyzes alpha-1,4-D-glucosidic bonds and slowly hydrolyzes alpha-1,6-D-glucosidic bonds, releasing glucose. A debranching enzyme, such as pullulanase or isoamylase, which rapidly hydrolyzes only the alpha-1,6-D-glucosidic bonds, releasing short chain amylose, has been suggested for use in conjunction with glucoamylase and alpha-amylase to improve the efficiency of production of high dextrose syrups. These syrups are starting materials in the manufacture of crystalline dextrose and high fructose corn syrup. See Maize, Recent Progress in Chemistry and Technology, pp. 157-179, Academic Press, Inc. (1982); and Slominska, L., et al., Starch/Starke, 11: 386-390 (1985).
Additionally, debranching enzymes (enzymes which release short chain amylose from starch) have been proposed for use in low calorie alcoholic beverage production to improve fermentability of branched starch fragments; in production of maltose from starch in conjunction with beta-amylase; in low DE maltodextrin (30-55 glucose units) production to induce proteins to aggregate in aqueous emulsions; and in enzymatic conversion of starch into a soluble syrup having a high quantity of disaccharides and trisaccharides. These debranching enzyme applications are directed to problems arising from the presence of branched starch or dextrin fragments following starch conversion processes. In each application, the debranching enzyme is employed in the complete conversion of starch to a variety of low molecular weight fragments such as sugars or maltodextrins. The thickening, adhesion and gelling characteristics of starch are lost.
The use of debranching enzymes to fully debranch starch, with hydrolysis of substantially all alpha-1,6-D-glucosidic bonds, so as to obtain pure, or amylopectin-free, low molecular weight amylose is taught in U.S. Pat. No. 3,730,840 to Sugimoto, et al, U.S. Pat. No. 3,881,991 to Kurimoto, et al, and U.S. Pat. No. 3,879,212 to Yoshida. These patents do not teach the conversion of starch to sugars and other soluble fragments. The object of these patents is to produce pure short chain amylose. The presence of any residual amylopectin is taught to be objectionable.
The background of enzyme-related starch technology does not suggest that useful starch compositions may be prepared by employing debranching enzymes to partially debranch the amylopectin component of the starch, yielding a mixture of short chain amylose, amylopectin and partially debranched amylopectin, with or without substantial conversion of the starch. The functional properties of the partially debranched starch of this invention are novel. Furthermore, nothing in the literature suggests the utility of an enzymatic process for partially debranching starch as a replacement, in whole, or in part, for processes that are commercially used to produce various modified starches. The enzymatic process offers significant advantages over other processes, particularly in food and cosmetic applications where the demand for "natural" products persists.
It is an object of this invention to provide starches which are partially enzymatically debranched and which possess a variety of properties which the untreated, branched starches do not possess. These properties are in addition to the expected rheological properties (i.e., thickening and adhesion) for which starches have traditionally been employed. These properties include, but are not limited to, fat-like textures ranging from oily to creamy to waxy, stable cloud formation, cold-water insoluble film formation, high strength gel formation, and thermoreversible gel formation in aqueous dispersions. A thermally reversible starch gel is one which melts upon heating and reforms upon cooling. Gels prepared from unmodified starches are not thermally reversible.
It is a further object of this invention to provide an enzymatic process for producing these starches, employing an alpha-1,6-D-glucanohydrolase, such as pullulanase, isoamylase or amylo-1,6-glucosidase.